The invention relates to a method for operating a hybrid drive train for a motor vehicle, wherein the hybrid drive train comprises an internal combustion engine and an electric motor as drive units, and wherein a parallel variable speed transmission with a first transmission input shaft, a second transmission input shaft and a transmission output shaft is disposed in the hybrid drive train, wherein the first transmission input shaft can be coupled to the transmission output shaft through at least a first gear and the second transmission input shaft can be coupled to the transmission output shaft by at least a second gear, wherein the internal combustion engine can be brought into operative engagement with the first transmission input shaft through a first clutch and can be brought into operative engagement with the second transmission input shaft through a second clutch, and wherein the electric motor is drivingly engaged with the second transmission input shaft.
Such a method is known from the book “7.LuK Colloquium 11./12. April 2002”, editor: LuK GmbH & Co., pages 252-254. Thus, for accelerating the motor vehicle from a stop, initially the gear “1” is shifted at the first transmission input shaft with the first and second clutch open, and the gear “2” is preselected at the second transmission input shaft. Then, the first clutch is closed, in order to transfer torque from the internal combustion engine to the drive wheels of the motor vehicle. Additionally, the drive wheels are driven by the electric motor through the second transmission input shaft and through the preselected gear “2” (boost function).
Alternatively, the gear “1” can be shifted at the first transmission input shaft with the first clutch open, and the gears of the second transmission input shaft can be shifted into neutral position. Then, the first and the second clutch are closed, in order to transfer a respective torque to the drive wheels of the motor vehicle by means of the internal combustion engine and the electric motor. The force flow from the electric motor thus runs through the second transmission input shaft, the second clutch, the input shaft of the combustion engine, the first clutch, the first transmission input shaft and the gear “1” to the transmission output shaft. Preselecting a gear on the second input shaft is performed with this shifting strategy while driving.
After the motor vehicle has reached a predetermined first speed, the internal combustion engine is shifted from first gear to the gear “2”. For this purpose, the second clutch is closed, while the first clutch is simultaneously opened and the motor vehicle continues to be driven by the electric motor, too.
Now gear “3” is preselected at the first transmission input shaft. After the motor vehicle has reached a predetermined second speed, the internal combustion engine is shifted from gear “2” to gear “3”. For this purpose, the first clutch is closed, while the second clutch is simultaneously opened and the motor vehicle continues to also be driven by the electric motor through the gear “2”, which is still shifted.
Now the gear “4” is preselected at the second transmission input shaft, while the second clutch continues to be open. Thus, initially the torque of the electric motor is reduced to zero. Subsequently, the gear “2” is brought into neutral position and then the gear “4” is shifted. Thereafter, torque is generated again by the electric motor and transferred through the preselected gear “4” to the drive wheels of the motor vehicle.
After the motor vehicle has reached a predetermined third speed, the internal combustion engine is shifted from gear “3” to gear “4”. For this purpose, the second clutch is closed, while the first clutch is simultaneously opened, and the motor vehicle continues to be also driven by the electric motor. When the motor vehicle is accelerated further, the shifting from the gear “4” into the gear “5” and from there into the gear “6” is performed in the same manner.
The method has the disadvantage that during a gear change on the second transmission input shaft, thus when shifting from gear “2” to gear “4”, and from gear “4” to gear “6”, no torque can be transferred from the electric motor to the drive wheels, since the partial transmission formed by the second gears is in neutral position for the moment. The loss of propulsion associated therewith is perceived as detrimental by the users of the motor vehicle.